[time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC serial port for ntpd

Bob Stewart bob at evoria.net
Wed Nov 9 18:28:11 EST 2016


Tom,
How many times have people posted here, on time-nuts, not to trust the trailing edge of a 1PPS pulse? 

Bob -----------------------------------------------------------------
AE6RV.com

GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info

      From: Tom Van Baak <tvb at LeapSecond.com>
 To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Wednesday, November 9, 2016 4:44 PM
 Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC serial port for ntpd
   
Bob,

The PIC's I use have essentially no jitter. If they generate a 1PPS the edge and the pulse width are perfect, down to picoseconds. The talk about "other stuff" and "priority" and "number of compares" and "ambiguity" is worrisome. It sounds like a design or coding flaw to me, like what happens when people try to do precise time with a high level language.

/tvb

----- Original Message ----- 
From: "Bob Stewart" <bob at evoria.net>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Wednesday, November 09, 2016 9:30 AM
Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC serial port for ntpd


I'd like to comment on the idea of measuring the width of the pulse. My experience with creating the 1PPS from an interrupt is that it's fairly straightforward how to do the set interrupt: The interrupt happens, you execute one, maybe two instructions to raise the output pin, and you leave the interrupt routine. But, resetting the pin is a different story. Unless you've got a lot of interrupt vectors to play with, the reset part of the 1PPS signal is delegated to a general purpose timing interrupt where you're doing a lot of other stuff - and it has a somewhat lower priority. So, you wind up doing a number of compares to see if you should reset the pin, which adds some ambiguity to the set/reset times.
Granted, I'm doing this on a general purpose PIC, but I have read comments about various receivers having some jitter on the reset side of their 1PPS pulse.
Bob

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